Method for transferring a useful layer to a carrier substrate

ABSTRACT

A method for transferring a useful layer to a carrier substrate comprises: joining a front face of a donor substrate to a carrier substrate along a bonding interface to form a bonded structure; annealing the bonded structure to apply a weakening thermal budget thereto and bring a buried weakened plane in the donor substrate to a defined level of weakening, the anneal reaching a maximum hold temperature; and initiating a self-sustained and propagating splitting wave in the buried weakened plane by applying a stress to the bonded structure to lead to the useful layer being transferred to the carrier substrate. The initiation of the splitting wave occurs when the bonded structure experiences a thermal gradient defining a hot region and a cool region of the bonded structure, the stress being applied locally in the cool region, and the hot region experiencing a temperature lower than the maximum hold temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/FR2020/050369, filed Feb. 26, 2020,designating the United States of America and published as InternationalPatent Publication WO 2020/188169 A1 on Sep. 24, 2020, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. 1902674, filed Mar. 15, 2019.

TECHNICAL FIELD

The present disclosure relates to the field of microelectronics. Inparticular, the present disclosure relates to a process for transferringa useful layer to a carrier substrate.

BACKGROUND

A process for transferring a useful layer 3 to a carrier substrate 4,shown in FIG. 1 , is known from the prior art; this process, described,in particular, in documents WO2005043615 and WO2005043616 and referredto as the “SMART CUT™” process, comprises the following steps:

-   -   forming a buried weakened plane 2 by implanting light species        into a donor substrate 1 so as to form a useful layer 3 between        this plane and a surface of the donor substrate;    -   next, joining the donor substrate 1 to a carrier substrate 4 to        form a bonded structure 5;    -   then applying a heat treatment to the bonded structure 5 in        order to weaken the buried weakened plane;    -   and lastly, initiating a splitting wave, with self-sustained        propagation of the splitting wave in the donor substrate 1 along        the buried weakened plane 2.

In this process, the species implanted at the level of the buriedweakened layer 2 initiate the development of microcavities. Theweakening heat treatment has the effect of promoting the growth andpressurization of these microcavities. By applying additional outsideforces (energy pulse) after the heat treatment, a splitting wave isinitiated in the buried weakened layer 2, which wave propagates in aself-sustained manner, resulting in the useful layer 3 being transferredthrough detachment at the level of the buried weakened plane 2. Such aprocess makes it possible, in particular, to decrease the roughness ofthe surface after transfer.

This process may be used to produce silicon-on-insulator (SOI)substrates. In this case, the donor substrate 1 and the carriersubstrate 4 are each formed of a silicon wafer, the standard diameter ofwhich is typically 200 mm, 300 mm or 450 mm for later generations.Either or both of the donor substrate 1 and the carrier substrate 4 aresurface-oxidized.

SOI substrates must comply with very stringent specifications. This isparticularly the case for the average thickness and the uniformity ofthickness of the useful layer 3. Complying with these specifications isnecessary for the semiconductor devices that will be formed in and onthis useful layer 3 to operate correctly.

In some cases, the architecture of these semiconductor devices requiresarranging SOI substrates exhibiting a very low average thickness of theuseful layer 3, for example, lower than 50 nm, and exhibiting very highuniformity of thickness for the useful layer 3. The expected uniformityof thickness may be about 5% at most, corresponding to variationstypically from +/−0.3 nm to +/−1 nm over the entire surface of theuseful layer 3. Even if additional finishing steps, such as etches orsurface-smoothing heat treatments, are carried out after the usefullayer 3 has been transferred to the carrier substrate 4, it is importantfor the morphological surface properties (in particular, uniformity ofthickness and surface roughness) to be as favorable as possible aftertransfer in order to ensure that the end specifications are met.

The applicant has observed that, when the splitting wave is initiatedafter the heat treatment at ambient temperature by applying an energypulse to the buried weakened plane 2, some useful layers 3 may include,after transfer, marbling-like irregular patterns due to local variationsin thickness, the amplitude of which is about a nanometer or half ananometer. This marbling may be distributed over the entirety of theuseful layer 3, or over only a portion thereof. It contributes to thenon-uniformity of the useful layer 3.

This type of non-uniformity in the thickness of the useful layer 3 isvery difficult to eliminate using the typical finishing techniques(etching, sacrificial oxidation, smoothing heat treatment, etc.) becausethese techniques are not effective in erasing irregular patterns of thisamplitude.

BRIEF SUMMARY

The present disclosure relates to a process for transferring a usefullayer to a carrier substrate and aims, in particular, to improve theuniformity of thickness of the transferred useful layers.

The present disclosure relates to a process for transferring a usefullayer to a carrier substrate, comprising the following steps:

-   -   a) providing a donor substrate including a buried weakened        plane, the useful layer being delimited by a front face of the        donor substrate and the buried weakened plane;    -   b) providing a carrier substrate;    -   c) joining the donor substrate, by its front face, to the        carrier substrate along a bonding interface so as to form a        bonded structure;    -   d) annealing the bonded structure in order to apply a weakening        thermal budget thereto and to bring the buried weakened plane to        a defined level of weakening, the anneal reaching a maximum hold        temperature; and    -   e) initiating a splitting wave in the buried weakened plane by        applying a stress to the bonded structure, the splitting wave        propagating in a self-sustained manner along the buried weakened        plane in order to result in the useful layer being transferred        to the carrier substrate.

The transfer process is noteworthy in that the initiation in step e)takes place when the bonded structure is experiencing a thermal gradientdefining a hot region and a cool region of the bonded structure, thestress being applied locally in the cool region, and the hot regionexperiencing a temperature lower than the maximum hold temperature.

According to other advantageous and non-limiting features of the presentdisclosure, taken alone or in any technically feasible combinations:

-   -   the thermal gradient is chosen to be between 20° C. and 100° C.,        preferably, between 60° C. and 90° C., more preferably, about        80° C.;    -   the maximum hold temperature is between 300° C. and 600° C.;    -   annealing step d) takes place in a heat treatment apparatus and        the initiation in step e) takes place when the bonded structure        exits the heat treatment apparatus;    -   the heat treatment apparatus is a horizontally or vertically        configured oven, suitable for batch-treating a plurality of        bonded structures;    -   the initiation in step e) takes place when the hot region of the        bonded structure reaches a temperature between 150° C. and 250°        C.;    -   the weakening thermal budget is between 40% and 95% of a        splitting thermal budget, the splitting thermal budget being        that which leads to spontaneous initiation of the splitting wave        in the buried weakened plane during the anneal;    -   the stress applied at the periphery of the bonded structure by        inserting a wedge at the bonding interface of the bonded        structure, between chamfered edges of the donor substrate and of        the carrier substrate, respectively, of the bonded structure;        and    -   the donor substrate and the carrier substrate are made of        monocrystalline silicon, and wherein the buried weakened plane        is formed by ion implantation of light species into the donor        substrate, the light species being chosen from hydrogen and        helium, or a combination of hydrogen and helium.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present disclosure will becomeapparent from the following detailed description of embodiments of thepresent disclosure, which description is given with reference to theaccompanying figures, in which:

FIG. 1 shows a process for transferring a thin layer according to theprior art;

FIG. 2 shows a transfer process according to the present disclosure;

FIGS. 3 a and 3 b show haze maps at the surface of useful layersincluding non-uniformities of thickness after transfer;

FIG. 4 shows a haze map at the surface of a useful layer transferredusing a transfer process according to the present disclosure;

FIG. 5 shows a step in a transfer process according to the presentdisclosure;

FIG. 6 is a graph showing the variation in the temperatures and in thethermal gradient seen by a bonded structure in a step in a transferprocess according to the present disclosure; and

FIG. 7 shows a step in a transfer process according to the presentdisclosure.

DETAILED DESCRIPTION

In the description, the same reference signs in the figures might beused for elements of the same type. The figures are schematicrepresentations, which, for the sake of legibility, are not to scale. Inparticular, the thicknesses of the layers along the z-axis are not toscale with respect to the lateral dimensions along the x- and y-axes;and the relative thicknesses of the layers with respect to one anotherare not necessarily respected in the figures. It should be noted thatthe coordinate system (x,y,z) of FIG. 1 applies to FIG. 2 .

The present disclosure relates to a process for transferring a usefullayer 3 to a carrier substrate 4. The useful layer 3 is named as suchbecause it is intended for use in the production of components in thefields of microelectronics or microsystems. The useful layer and thecarrier substrate may vary in nature depending on the target componenttype and target application. Since silicon is currently the mostcommonly used semiconductor material, the useful layer and the carriersubstrate may be made of, in particular, monocrystalline silicon, butare, of course, not limited to this material.

The transfer process according to the present disclosure first comprisesa step a) of providing a donor substrate 1, from which the useful layer3 will be taken. The donor substrate 1 includes a buried weakened plane2 (FIG. 2 -a)). The latter is advantageously formed by ion-implantinglight species into the donor substrate 1 at a defined depth. The lightspecies are chosen, preferably, from hydrogen and helium, or acombination of hydrogen and helium, since these species promote theformation of microcavities around the defined implantation depth,resulting in the buried weakened plane 2.

The useful layer 3 is delimited by a front face 1 a of the donorsubstrate 1 and the buried weakened plane 2.

The donor substrate 1 may be formed of at least one material chosen fromsilicon, germanium, silicon carbide, IV-IV, III-V or II-VI semiconductorcompounds and piezoelectric materials (for example, LiNbO₃, LiTaO₃,etc.). It may further include one or more surface layers arranged on thefront face 1 a and/or on the back face 1 b thereof, which may be of anynature, for example, dielectric.

The transfer process also comprises a step b) of providing a carriersubstrate 4 (FIG. 2 -b)).

The carrier substrate may be formed, for example, of at least onematerial chosen from the silicon, silicon carbide, glass, sapphire,aluminum nitride or any other material that might be available insubstrate form. It may also include one or more surface layers of anynature, for example, dielectric.

As mentioned above, one advantageous application of the transfer processaccording to the present disclosure is the production of SOI substrates.In this particular case, the donor substrate 1 and the carrier substrate4 are made of monocrystalline silicon, and either or both of thesubstrates include a surface layer of silicon oxide 6 on the front facethereof.

The transfer process next comprises a step c) of joining the donorsubstrate 1, by its front face 1 a, to the carrier substrate 4 along abonding interface 7 so as to form a bonded structure 5 (FIG. 2 -c)).

The joining operation may be carried out using any known method, inparticular, by direct bonding by molecular adhesion, bythermocompression, by electrostatic bonding, etc. These well-knownprior-art techniques will not be described in detail here. However, itis recalled that, prior to joining, the donor substrate 1 and thecarrier substrate 4 will have undergone surface-activation and/orcleaning sequences in order to ensure the quality of the bondinginterface 7 in terms of defects and bonding energy.

In the transfer process according to the present disclosure, a step d)of annealing the bonded structure 5 is then carried out in order toapply a weakening thermal budget to the bonded structure 5 and to bringthe buried weakened plane to a defined level of weakening (FIG. 2 -d)).The time/temperature pairs applied during the anneal determine thethermal budget to which the bonded structure 5 is subjected.

The range of temperatures over which the anneal may be carried out forthis operation of weakening the buried plane 2 depends primarily on thetype of bonded structure 5 (homostructure or heterostructure) and on thenature of the donor substrate 1.

By way of example, in the case of a donor substrate 1 and a carriersubstrate 4 made of silicon, the anneal in step d) reaches a maximumhold temperature that is typically between 200° C. and 600° C.,advantageously between 300° C. and 500° C. and even more advantageouslybetween 350° C. and 450° C. More generally, the maximum hold temperaturecould, for materials used for the donor substrate 1 and/or for thecarrier substrate 4 other than silicon, typically be between 200° C. and800° C.

The anneal may include a temperature ramp-up (typically between 200° C.and the maximum hold temperature) and a hold at the maximum temperature.In general, the duration of such an anneal will be between a few tens ofminutes and several hours, depending on the maximum hold temperature ofthe anneal.

The level of weakening of the buried weakened plane 2 is defined by thearea occupied by the microcavities present in the buried weakened layer2. In the case of a donor substrate 1 made of silicon, this areaoccupied by the microcavities may be characterized by infraredmicroscopy.

The level of weakening may range from a low level (<1%, below thedetection threshold of the characterizing instruments) up to more than80%, depending on the thermal budget applied to the bonded structure 5during the anneal.

The weakening thermal budget is always kept below a splitting thermalbudget for which spontaneous initiation of the splitting wave in theburied weakened plane 2 is obtained during the anneal. Preferably, theweakening thermal is between 40% and 95% of the splitting thermalbudget.

It should be noted that the heat treatment apparatus 20 in which theanneal in step d) is performed is, preferably, a horizontally orvertically configured oven, suitable for batch-treating a plurality ofbonded structures 5.

In the transfer process according to the present disclosure, a step e)of initiating a splitting wave along the buried weakened plane 2 is nextcarried out by applying a stress to the bonded structure 5 (FIG. 2 -e)).After initiation, the splitting wave propagates in a self-sustainedmanner, resulting in the separation of the bonded structure 5 at theburied weakened plane 2. Self-sustained propagation means that onceinitiated, the spitting wave propagates by itself, without theapplication of external stress, throughout the entire extent of theburied weakened plane 2, so that the useful layer 3 is completelydetached from the donor substrate 1 and transferred to the carriersubstrate 4. A transferred assembly 5 a and the remainder 5 b of thedonor substrate 1 are thus obtained (FIG. 2 -f)).

Advantageously, the stress is local and applied at the periphery of thebonded structure 5. It may be caused mechanically or by any other means,such as, for example, local heating by means of a laser or energytransfer by means of ultrasound.

Preferably, a local mechanical load may be applied by inserting a wedgeat the bonding interface 7 of the bonded structure 5, between chamferededges of the donor substrate 1 and of the carrier substrate 4,respectively, of the bonded structure 5. This results in tensile strainbeing generated in the buried weakened plane 2.

It is recalled that, by applying the transfer process of the prior artmentioned in the introduction, which involves mechanically initiatingthe splitting wave at ambient temperature, the applicant has observedmarbling-like irregular patterns that negatively affect the uniformityof thickness of the useful layer 3 after transfer. The applicant hasidentified that these irregular patterns are related to an instabilityin the propagation of the splitting wave due to insufficient energystored in the [bonded structure 5/buried weakened layer 2] system.

To overcome these problems and to improve the uniformity of thickness ofthe useful layer 3 after transfer, the transfer process according to thepresent disclosure envisages that the splitting wave is initiated, instep e), by applying an external stress to the buried weakened plane 2when the bonded structure 5 is experiencing a thermal gradient defininga hot region and a cool region of the bonded structure 5. According tothe present disclosure, the external stress is applied locally in thecool region of the bonded structure 5. The maximum temperatureexperienced by the hot region is always lower than the maximum holdtemperature of the anneal.

Advantageously, the initiation in step e) is chosen to take place whenthe bonded structure 5 is experiencing a thermal gradient that is largerthan 20° C. and smaller than 100° C., in particular, between 60° C. and90° C., preferably, about 80° C.

This particular configuration makes it possible to limit variations inthe thickness of the useful layer 3, which may appear in the form ofregular or irregular patterns with an amplitude of about a nanometer orhalf a nanometer.

Specifically, it has been identified that the non-uniformities ofthickness caused by the propagation of the splitting wave may have twodistinct origins: first, excess energy released at the start ofpropagation (creating regular patterns with a pitch of about 1 cm);second, a lack of energy released at the end of propagation, generatinginstability in the split (resulting in an irregular pattern). Now, theenergy released by splitting is directly proportional to the temperatureof the material in which it propagates. Thus, if it is initiated in thehot region of the bonded structure 5, the split will initially release alot of energy (which is disadvantageous in terms of regular patterns)and then, at the end of propagation, release less energy (which isdisadvantageous in terms of irregular patterns). By initiating in thecool region of the bonded structure 5, the split initially releaseslittle energy (which is advantageous in terms of regular patterns) then,at the end of propagation, releases more energy (which is advantageousin terms of irregular patterns).

By way of example, FIGS. 3 a and 3 b show haze maps of useful layers 3after, respectively, spontaneous splitting taking place during annealingand mechanical splitting initiated at ambient temperature after aweakening anneal. It should be noted that the aforementioned mechanicalsplitting operation was initiated by applying a local mechanical load tothe bonded structure 5 and generating strain in the buried weakenedlayer 2. In each case, (regular or irregular) patterns, which negativelyaffect the uniformity of thickness of the useful layer 3 after transfer(between 0.5 nm and 1.5 nm in amplitude), are observed. These patternshave been made apparent by measurement of haze, which corresponds to theintensity of the light scattered by the surface of the useful layer 3,using the SURF SCAN™ inspection tool by KLA-Tencor. In FIG. 3 a(spontaneous splitting), regular, periodic patterns with a wavelength ofabout 2 cm give rise to a local non-uniformity in the haze signal of 4%.In FIG. 3 b (mechanical splitting at ambient temperature), irregularpatterns give rise to a local non-uniformity in the haze signal of morethan 12%.

FIG. 4 shows a haze map at the surface of a useful layer 3 transferredusing a transfer process according to the present disclosure: in thisinstance, after splitting is initiated when the bonded structure 5 isexperiencing a thermal gradient of about 80° C. by applying a localmechanical load to the cool region of the bonded structure 5, generatingstrain in the buried weakened plane 2. No pattern, whether regular orirregular and marbling-like, is present, and the local non-uniformity inthe haze signal does not exceed 2%. The uniformity of thickness of theuseful layer 3 is thus substantially improved.

Generally speaking, a splitting wave initiated in the cool region of thebonded structure 5, when there is a thermal gradient larger than 20° C.and smaller than 100° C. across the bonded structure 5, leads to a highdegree of uniformity of thickness for the useful layer 3 after transfer.

According to one advantageous embodiment of the present disclosure, thesplitting wave is initiated when the bonded structure 5 exits the heattreatment apparatus 20 in which the anneal in step d) was performed:there is generally a thermal gradient across the bonded structure 5 whenit exits the apparatus (FIGS. 5 and 6 ). This gradient is generally dueto the geometry of the oven 20 and the presence of a system for holdingthe bonded structures 5, which affects heat dissipation. For example, inthe case of a horizontally configured oven 20 in which the bondedstructures 5 are placed vertically into cassettes 22 that are borne by acharge shovel 21, it is observed that the lower region B (cod region) ofthe bonded structures 5 (i.e., that closest to the holding system formedby the cassette 22 and the charge shovel 21) is cooler than the upperregion H of the bonded structures 5. The local mechanical load is thenapplied in the lower region B (cool region) of the bonded structure 5,according to the present disclosure.

Preferably, the initiation in step e) takes place at the exit of theheat treatment apparatus 20 when the hot region of the bonded structure5 is at a temperature between 150° C. and 250° C., preferably, about200° C. When the splitting wave is initiated within the aforementionedtemperature range, the energy stored in the system (bonded structure5+buried weakened plane 2), and, in particular, the energy stored in theburied weakened plane 2 due to the presence of pressurized gaseousspecies in the microcavities, is suitable for effective self-sustainedpropagation, further improving the surface state of the useful layer 3after transfer.

Exemplary Application:

The transfer process according to the present disclosure may be used forthe production of SOI substrates, the useful layer 3 of which is verythin, in particular, between a few nanometers and 50 nm.

The example used is that of donor substrate 1 and carrier substrate 4and made of monocrystalline silicon, each taking the form of a 300mm-diameter wafer. The donor substrate is covered with a layer ofsilicon oxide 6 with a thickness of 50 nm. The buried weakened plane 2is formed in the donor substrate 1 by co-implanting hydrogen and heliumions under the following conditions:

-   -   H: implantation energy 38 keV, dose 1E16 H/cm²; and    -   He: implantation energy 25 keV, dose 1E16 He/cm².

The buried weakened plane 2 is located at a depth of about 290 nm fromthe front face 1 a of the donor substrate 1. It delimits, with thesilicon oxide layer 6, a useful layer 3 of about 240 nm.

The donor substrate 1 is joined to the carrier substrate 4 by directbonding by molecular adhesion, to form the bonded structure 5. Prior tojoining, the donor substrate 1 and the carrier substrate 4 will haveundergone known surface-activation and/or cleaning sequences in order toensure the quality of the bonding interface 7 in terms of defects andbonding energy.

A horizontally configured oven 20 is used to perform the batch-annealingof a plurality of bonded structures 5 such as described above. This typeof heat treatment apparatus 20 comprises a charge shovel 21, which bearscassettes 22 into which the bonded structures 5 are placed (FIG. 7 ).The charge shovel 21 moves between an entered position, in which thebonded structures 5 are inside the oven 20, and an exited position, inwhich they are outside the oven 20.

A system of wedges 10 may be positioned on each cassette 22, under thebonded structures 5. The charge shovel 21 moves to the entered positionfor the anneal to be performed. The anneal comprises a temperatureramp-up from 200° C. to 380° C., a hold at 380° C. for two minutes and atemperature ramp-down to 225° C.

Upon completion of the anneal, the charge shovel 21 moves to its exitedposition.

As illustrated in FIG. 6 , as soon as each bonded structure 5 has exitedthe oven 20, the temperature it experiences decreases and a thermalgradient arises. The cool region of the bonded structure 5 is located ina bottom region B, i.e., close to the charge shovel 21 and the cassette22. Each bonded structure 5 will move on to an exit zone 23 in which itwill experience a thermal gradient that is, preferably, between 60° C.and 90° C., and, in particular, about 80° C.+/−10° C., or even 80°C.+/−5° C. In this exit zone 23, a pressing device 11 located above thebonded structures 5 will exert a pressing force on each bonded structure5 successively, such that the wedge 10 thereunder (in the cool region)will be inserted, at the bonding interface 7, between the chamferededges of the joined substrates of the bonded structure 5 (FIG. 7 ). Theinsertion of the wedge 10 generates local tensile strain at the site ofthe buried weakened plane 2, allowing the splitting wave to be initiatedin the cool region of each bonded structure 5 in succession as they passbeneath the pressing device 11.

Of course, tools other than the assembly formed by the system of wedges10 and the pressing device 11 could be implemented to initiate thesplitting wave in the bonded structures 5 in accordance with the presentdisclosure.

The splitting wave is thus initiated for each bonded structure in itscool region (bottom region B) when it is experiencing a thermal gradientof about 80° C. (about 70° C. in the example of FIG. 6 ).

In the example of FIG. 6 , the exit zone 23 in which the initiation ofthe splitting wave takes place corresponds to a zone in which eachbonded structure 5 is subjected, in its coolest region (bottom regionB), to a temperature of about 130° C., its central region C experiencingan intermediate temperature of about 180° C. and its top region Hexperiencing a temperature of about 200° C.

Following the self-sustained propagation of the splitting wave, what isobtained, after transfer, is the SOI substrate (transferred assembly 5a) and the remainder 5 b of the donor substrate. A very high degree ofuniformity of thickness is obtained for the transferred useful layers 3(similar to the result of FIG. 4 ).

Finishing steps applied to the transferred assemblies 5 a comprisechemical cleaning operations and at least one high-temperature smoothingheat treatment. Upon completion of these steps, the SOI substratesinclude a useful layer 3 with a thickness of 50 nm, the finalnon-uniformity of thickness of which is about 0.45 nm. It should benoted that, in comparison, SOI substrates of which the useful layer 3includes regular or irregular patterns after splitting may exhibit finalnon-uniformities of thickness that are greater than or equal to 0.7 nm.

Of course, the present disclosure is not limited to the describedimplementations and examples, and variant embodiments may be introducedthereinto without departing from the scope of the invention as definedby the claims.

The invention claimed is:
 1. A method for transferring a useful layer toa carrier substrate, comprising the following steps: a) providing adonor substrate including a buried weakened plane, the useful layerbeing delimited by a front face of the donor substrate and the buriedweakened plane; b) providing a carrier substrate; c) joining the donorsubstrate, by its front face, to the carrier substrate along a bondinginterface so as to form a bonded structure; d) annealing the bondedstructure to apply a weakening thermal budget thereto and to bring theburied weakened plane to a defined level of weakening, the annealreaching a maximum hold temperature; and e) initiating a splitting wavein the buried weakened plane by applying a stress to the bondedstructure, the splitting wave propagating in a self-sustained manneralong the buried weakened plane to result in the useful layer beingtransferred to the carrier substrate; wherein the initiation in step e)takes place when the bonded structure is experiencing a thermal gradientdefining a hot region and a cool region of the bonded structure, thestress being applied locally in the cool region, and the hot regionexperiencing a temperature lower than the maximum hold temperature. 2.The method of claim 1, wherein the thermal gradient is between 20° C.and 100° C.
 3. The method of claim 2, wherein the thermal gradient isbetween 60° C. and 90° C.
 4. The method of claim 3, wherein the thermalgradient is about 80° C.
 5. The method of claim 2, wherein the maximumhold temperature is between 300° C. and 600° C.
 6. The method of claim5, wherein the annealing step d) takes place in a heat treatmentapparatus and the initiation in step e) takes place when the bondedstructure exits the heat treatment apparatus.
 7. The method of claim 6,wherein the heat treatment apparatus is a horizontally or verticallyconfigured oven, configured for batch-treating a plurality of bondedstructures.
 8. The method of claim 7, wherein the initiation in step e)takes place when the hot region of the bonded structure reaches atemperature between 150° C. and 250° C.
 9. The method of claim 8,wherein the weakening thermal budget is between 40% and 95% of asplitting thermal budget, the splitting thermal budget leading tospontaneous initiation of the splitting wave in the buried weakenedplane during the anneal.
 10. The method of claim 9, wherein the stressis applied at the periphery of the bonded structure by inserting a wedgeat the bonding interface of the bonded structure, between chamferededges of the donor substrate and of the carrier substrate, respectively,of the bonded structure.
 11. The method of claim 10, wherein the donorsubstrate and the carrier substrate comprise monocrystalline silicon,and wherein the buried weakened plane is formed by ion implantation oflight species into the donor substrate, the light species being chosenfrom among hydrogen and helium, or a combination of hydrogen and helium.12. The method of claim 1, wherein the maximum hold temperature isbetween 300° C. and 600° C.
 13. The method of claim 1, wherein theannealing step d) takes place in a heat treatment apparatus and theinitiation in step e) takes place when the bonded structure exits theheat treatment apparatus.
 14. The method of claim 13, wherein the heattreatment apparatus is a horizontally or vertically configured oven,configured for batch-treating a plurality of bonded structures.
 15. Themethod of claim 1, wherein the initiation in step e) takes place whenthe hot region of the bonded structure reaches a temperature between150° C. and 250° C.
 16. The method of claim 1, wherein the weakeningthermal budget is between 40% and 95% of a splitting thermal budget, thesplitting thermal budget leading to spontaneous initiation of thesplitting wave in the buried weakened plane during the anneal.
 17. Themethod of claim 1, wherein the stress is applied at the periphery of thebonded structure by inserting a wedge at the bonding interface of thebonded structure, between chamfered edges of the donor substrate and ofthe carrier substrate, respectively, of the bonded structure.
 18. Themethod of claim 1, wherein the donor substrate and the carrier substratecomprise monocrystalline silicon, and wherein the buried weakened planeis formed by ion implantation of light species into the donor substrate,the light species being chosen from among hydrogen and helium, or acombination of hydrogen and helium.